The present invention relates to engines in general, and more particularly to improvements in cylinder and piston units of internal combustion engines, especially diesel engines. Still more particularly, the invention relates to improvements in composite pistons of the type disclosed in the aforementioned commonly owned U.S. Pat. No. 4,593,660.
The patent discloses a composite piston having a first section which serves to seal the interior of the cylinder from fuel and gaseous combustion products, and a second or inner section which serves to guide the piston in the cylinder. The second section is designed to take up lateral stresses, and the two sections are connected to the respective end portion of the connecting rod by a common piston pin. An advantage of a composite piston is that its sections can be made of different materials. For example, the first section can be made of iron and the second section can be made of aluminum or an aluminum alloy. The sealing and thermal insulating properties of a section which is made of iron are highly satisfactory. On the other hand, a section which is made of aluminum or an aluminum alloy exhibits highly satisfactory heat conductivity and is readily slidable along the internal surface of the cylinder. Such optimum selection of the materials of the two sections contributes to longer useful life and stability of the composite piston, and the first section can be acted upon by fluids (particularly gases) at elevated pressure, i.e., at a pressure which is to be expected in a modern diesel engine. In order to further enhance the stability of the first section, while at the same time reducing the weight of such section, the first section is preferably dimensioned and configurated in a manner as disclosed in the U.S. Pat. No. 4,593,660, namely so that imaginary straight lines which extend from the exposed side of the head of the first section to the piston pin bushing of the connecting rod do not intersect the exposed surfaces of the head and the portion (preferably in the form of a hollow conical frustum) which connects the head with the piston pin bosses of the first section.
Stresses which develop as a result of the application of elevated gas pressure to the head of the first piston section are transmitted to the piston pin exclusively by the bosses of the first section. Thus, the bosses are compelled to take up and to transmit extremely large forces which are likely to cause breakage, distortion and/or misalignment of the bosses with attendant destruction of the entire piston-cylinder unit or seizing of the piston pin. It has been found that a mere addition of material to reinforce the bosses does not suffice to ensure the retention of bosses in optimum positions with reference to the piston pin, expecially in a modern diesel engine wherein the head of the first section of the composite piston is subjected to extremely high pressures. Moreover, a mere addition of bulk and weight to the bosses is undesirable on the additional ground that the trend is toward the utilization of lightweight pistons, even in diesel engines of the aforediscussed design wherein the head of the first piston section must stand elevated pressures.
The bosses of heretofore known piston sections are subjected to elevated static pressures as well as to pronounced thermal stresses. The static pressures develop as a result of the application of elevated gas pressure to the head of the first section of the composite piston which results in a tendency of the bosses to migrate in the axial direction of the piston pin and away from each other. Attempts to simply extend the bosses in the circumferential direction of the cylinder have failed because of the pronounced temperature differential between the regions near to and the regions remote from the aforementioned connecting portion between the head and the bosses of the first section of the composite piston. The temperature of the bosses increases very rapidly in a direction from the piston pin toward the head of the first section. In the absence of any action to the contrary, static forces which develop as a result of the application of elevated gas pressures to the head of the first section will tend to spread the bosses apart and to establish highly localized points of transmission of elevated pressure between the inner portions of the bosses and the peripheral surface of the pin. This can entail serious damage to or destruction of small portions of the surfaces surrounding the openings which are provided in the bosses for the piston pin. When the pressure upon the exposed surface of the head decreases, the bosses are acted upon primarily by thermal stresses due to pronounced temperature differences between the portions nearer to and the portions more distant from the head so that the free end portions of the bosses tend to move toward each other, i.e., the inclination of the bosses with reference to the axis of the pin changes and the pin is likely to jam. Excessive stressing is then shifted from the innermost to the outermost portions of internal surfaces which surround the openings of the bosses.
The piston pin should retain its freedom of movement (rotation) relative to the piston sections under any and all circumstances because any interference with such freedom of rotation will entail the development of pronounced localized stresses upon relatively small portions of the peripheral surface of the pin. This can destroy the layers of lubricant which coat the periphery of the pin and the internal surfaces of the bosses. It is equally important to ensure that the piston pin be free to turn relative to the piston pin bushing of the connecting rod. On the other hand, the piston pin must be received in the bushing and in the bosses with a minimum of play so that even minute changes in mutual spacing of the bosses can entail an elimination of such play with the aforediscussed undesirable consequences as regards seizing of the pin, destruction of the film of lubricant and/or material damage to and destruction of the pin, bosses and/or bushing. At any rate, the useful life of the bosses is unduly affected if they are free or compelled to travel axially of the piston pin toward or away from each other (depending upon whether they are subjected to thermal stresses or static pressures). As a rule, the clearance between the pin on the one hand and the internal surfaces of the bushing of the connecting rod and the bosses of the first section of the composite piston on the other hand is less than one thousandth of the diameter of the pin.